We're so, so close to the 100th episode of Arrow that we can taste it. The synopsis for the new episode is a bit cryptic (apparently Oliver Queen is trapped in some kind of alternate reality where he never got on the Queen's Gambit), but Stephen Arnell, one of the writers behind the show, is able to give a little bit of insight into what to expect in the huge multi-crossover episode.

First off, you have to realize that putting together this crossover was nothing short of madness. As Arnell describes in an interview with Collider:

"[They] handed me this big piece of paper that was just five straight weeks of where I was going each day. It was hard work, I mean I didn't have a day off for five weeks, but there was a spirit of teamwork this year that made it palatable."

But the wildest hint has to do with the seemingly experimental nature of the new episode, which might be the biggest crossover event in CW history. It's also the most complicated, thanks to the combination of aliens, time-travel (hey, Flashpoint!), cross-dimensional travel, and super-powered aliens (both friend and foe). Here's how Arnell describes the 100th episode:

"We utilize the fantastical elements that we bring in on The Flash and carry through to Legends of Tomorrow with the present day storyline to create something for the 100th episode of the show that is not a flashback, it's not a flash forward, it's something else entirely. It basically gives us license to do whatever we want and to show some things that we'll never be able to show on the show, to have some characters interact that will never be able to interact on the show again (probably)."

This is a pretty transparent hint that Ollie is probably going to meet up with this parents again—and possibly get some closure. Our best guess is that the episode is somehow connected to a Black Mercy-type situation, which we've seen before on Supergirl, but in the interview Arnell calls it "virtual reality".

Some fans were less than enthused about the Supergirl beginning to the crossover (not much Dominator-related stuff happened, except Barry appearing and asking for help), but the Flash episode was unexpectedly poignant. We'll see if Arnell and the team can pull of Arrow next.

Tune in tonight on the CW to catch Arrow's hundredth episode, Invasion!